It is well known in the art that polymers can be prepared by anionic polymerization employing organolithium compounds such as butyllithium. It is also known in the art that such polymers can be prepared and functionalized using particular initiators that impart a functional group on the polymer.
For example, it is desirable to produce elastomeric compounds exhibiting reduced hysteresis characteristics. Such elastomers, when used to form articles such as tires, power belts and the like, will show an increase in rebound, a decrease in rolling resistance, and will have less heat build-up when mechanical stresses are applied.
It is believed that hysteretic power loss results from that section of the polymer chain from the last cross-link of the vulcanizate to the end of the polymer chain. This free end cannot be involved in an efficient, elastically recoverable process, and as a result, any energy transmitted to this section of the vulcanizate is lost as heat.
One method of reducing such hysteretic power loss is to provide the polymer chain with a functional group that will interact with other substances compounded with the vulcanizate, e.g., carbon black, and thereby serve to stabilize that section of the vulcanizate between the end of the polymer chain and the last cross-link. This functional group can, for example, be achieved by anionically polymerizing the polymer with a cyclic lithioamine initiator, which technique is generally described in U.S. Pat. No. 5,329,005.
In the art of manufacturing rubber, temperatures during polymerization and processing can reach 50-120.degree. C. At these temperatures, some cyclic lithioamine functional groups can undergo elimination of the amine, resulting in a disruption of the polymerization process and/or loss of the polymer-bound amino content.
To overcome this shortcoming, U.S. Pat. No. 5,496,940 teaches cyclic amine alkyllithium compounds have been found to exhibit better heat stability during polymerization. The resulting polymers also exhibit a greater degree of heat stability. Preferred anionic initiators taught therein include hexamethyleneiminopropyllithium, hexamethyleneiminohexyllithium and dodecamethyleneiminopropyllithium.
Although many functional groups have been contemplated that would impart hysteretic loss reducing properties to vulcanizates, the initiators from which the functional groups ultimately derive must be soluble in solvents conducive to anionic polymerization. To address this issue, U.S. Pat. No. 5,332,810 teaches an anionic polymerization initiator that is soluble in acyclic alkane solvents. This particular initiator is the reaction product of an amine, an organolithium and a solubilizing component. The solubilizing component may be a hydrocarbon, an ether, an amine, or a mixture thereof.
Likewise, U.S. Pat. No. 5,578,542 teaches a mixed amine system, where otherwise non-soluble metal amides are rendered soluble when mixed with other metal amides. It has been found that such mixed amine systems increase solubility by about two-fold, thereby delaying the onset of visible precipitation for a period of days, and even weeks, at room temperature in hexanes.
U.S. Pat. No. 5,550,203 teaches an anionic polymerization initiator that is chain extended. The chain extension results in improved solubility of the initiator in hydrocarbon solvents and an increased rate of polymerization when Such initiators are employed. It has been found, however, that refrigeration of these initiators within a hydrocarbon solvent is required for storage because decomposition of the initiator can occur.